Revert "Remove NullPointerException."

tools/utils/vim/syntax/dart.vim

 " Vim syntax file " Language: Dart
 " Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 " for details. All rights reserved. Use of this source code is governed by a
 " BSD-style license that can be found in the LICENSE file.
 
 " Quit when a syntax file was already loaded
 if !exists("main_syntax")
   if version < 600
     syntax clear
   elseif exists("b:current_syntax")
@@ skipping 16 matching lines @@
 syn keyword dartOperator       new is in factory const
 syn match   dartOperator       "+=\=\|-=\=\|*=\=\|/=\=\|%=\=\|\~/=\=\|<<=\=\|>>=\=\|[<>]=\=\|===\=\|\!==\=\|&=\=\|\^=\=\||=\=\|||\|&&\|\[\]=\=\|=>\|!\|\~"
 syn keyword dartType           void var final bool int double num dynamic
 syn keyword dartCommonInterfaces String Object Math RegExp Date
 syn keyword dartInterfaces       Collection Comparable Completer Duration Function Future HashMap HashSet Iterable Iterator LinkedHashMap List Map Match Options Pattern Queue Set Stopwatch StringBuffer TimeZone
 syn keyword dartInterfaces_DEPRECATED       Dynamic
 syn keyword dartErrors         AssertionError TypeError FallThroughError
 syn keyword dartStatement      return
 syn keyword dartStorageClass   static abstract
 syn keyword dartExceptions     throw try catch finally
-syn keyword dartExceptions     Error FormatException Exception ExpectException FutureAlreadyCompleteException FutureNotCompleteException ArgumentError IllegalJSRegExpException IntegerDivisionByZeroException NoSuchMethodError NullThrownError OutOfMemoryError RangeError StackOverflowException StateError UnimplementedError UnsupportedError
+syn keyword dartExceptions     Error FormatException Exception ExpectException FutureAlreadyCompleteException FutureNotCompleteException ArgumentError IllegalJSRegExpException IntegerDivisionByZeroException NoSuchMethodError NullPointerException OutOfMemoryError RangeError StackOverflowException StateError UnimplementedError UnsupportedError
 syn keyword dartExceptions_DEPRECATED     BadNumberFormatException
 syn keyword dartAssert         assert
 syn keyword dartClassDecl      extends implements interface
 " TODO(antonm): check if labels on break and continue are supported.
 syn keyword dartBranch         break continue nextgroup=dartUserLabelRef skipwhite
 syn keyword dartKeyword        get set operator call equals negate external
 syn match   dartUserLabelRef   "\k\+" contained
 syn match   dartVarArg         "\.\.\."
 
 " TODO(antonm): consider conditional highlighting of corelib classes.
@@ skipping 71 matching lines @@
   unlet main_syntax
 endif
 
 let b:spell_options="contained"
 
 " Enable automatic indentation (2 spaces)
 set expandtab
 set shiftwidth=2
 set softtabstop=2
 set cindent